The invention relates to a downhole tool such as a well-logging tool, and more particularly to a tool of the measure-while-drilling (MWD) type.
When oil wells or other boreholes are being drilled it is frequently necessary to determine the orientation of the drilling tool so that it can be steered in the correct direction. Additionally, information may be required concerning the nature of the strata being drilled, or the temperature or pressure at the base of the borehole, for example. There is thus a need for measurements of drilling parameters, taken at the base of the borehole, to be transmitted to the surface.
One method of obtaining at the surface the data taken at the bottom of the borehole is to withdraw the drill string from the hole, and to lower measuring instrumentation including an electronic memory system down the hole. The relevant information is encoded in the memory to be read when the instrumentation is raised to the surface. Among the disadvantages of this method are the considerable time, effort and expense involved in withdrawing and replacing the drill string. Furthermore, updated information on the drilling parameters is not available while drilling is in progress.
A much-favoured alternative is to use a measure-while-drilling tool, wherein sensors or transducers positioned at the lower end of the drill string continuously or intermittently monitor predetermined drilling parameters and the tool transmits the appropriate information to a surface detector while drilling is in progress. Typically, such MWD tools are positioned in a cylindrical drill collar close to the drill bit, and use a system of telemetry in which the information is transmitted to the surface detector in the form of pressure pulses through the drilling mud or fluid which is circulated under pressure through the drill string during drilling operations. Digital information is transmitted by suitably timing the pressure pulses. The information is received and decoded by a pressure transducer and computer at the surface.
The drilling mud or fluid is used to cool the drill bit, to carry chippings from the base of the bore to the surface and to balance the pressure in the rock formations. Drilling fluid is pumped at high pressure down the centre of the drill pipe and through nozzles in the drill bit. It returns to the surface via the annulus between the exterior of the drill pipe and the wall of the borehole.
In a number of known MWD tools, a negative pressure pulse is created in the fluid by temporarily opening a valve in the drill collar to partially bypass the flow through the bit, the open valve allowing direct communication between the high pressure fluid inside the drill string and the fluid at lower pressure returning to the surface via the exterior of the string. However, the high pressure fluid can cause serious wear on the valve, and often pulse rates of only up to about 1 pulse per second have been achieved by this method. Alternatively, a positive pressure pulse can be created by temporarily restricting the flow through the downpath within the drill string by partially blocking the downpath.
U.S. Pat. No. 4,914,637 (Positec Drilling Controls Ltd) discloses a number of embodiments of MWD tool having a pressure modulator for generating positive pressure pulses. The tool has a number of blades equally spaced about a central body, the blades being split in a plane normal to the longitudinal axis of the body to provide a set of stationary half-blades, and a set of rotary half-blades. A temporary restriction in the fluid flow is caused by allowing the rotary half-blades to rotate through a limited angle, so that they are out of alignment with the stationary half-blades, the rotation being controlled by a solenoid-actuated latching means. In one embodiment, the drilling fluid is directed through angled vanes in front of the split blades in order to impart continuous torque to the rotary half-blades, such that the rotary half-blades rotate through a predetermined angle in the same direction each time the latch is released, thus being rotated successively into and out of alignment with the stationary half-blades.
The provision of angled driving vanes or blades upstream of the pulse-generating rotary half-blades is a generally convenient way of providing the necessary torque to the rotary half-blades to enable them to rotate and thus generate the pulses. We have found, however, that this arrangement can give rise to certain problems. In particular, as the flow of drilling fluid acts on the driving blades to provide the required driving force, an equal and opposite force is exerted on the fluid which, as a result, develops a swirling motion. The swirling motion of the fluid then tends to impair the operation of the downstream pulse-generating half-blades. In particular, the swirl of the fluid acts on the half-blades in the direction opposite to that in which the half-blades are being driven to generate the pressure pulses. Clearly, this may impede or even prevent the movement of the half blades, and thus the generation of the pulses.